PISKUNOV , A. G. Power of open $\sigma$-compact sets in the space of non-compact subgroups of the topological group. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 40, n. 6, p. 803–807, 1988. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/9297. Acesso em: 5 dec. 2025.